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Old 03-25-2008, 12:56 AM   #1
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Default A Guide to Subwoofers (Part I): Characteristics, Placement, & Adjustments


Prepared by Big Daddy

For multiple subwoofers, check A Guide to Subwoofers (Part II): Standing Waves & Room Modes and A Guide to Bass Management. For subwoofer problems, read DIY Subwoofers Building & Repairing thread.

Subwoofers are becoming more important to the home theater experience. They are designed for reproducing the lowest audio frequencies around 120Hz and lower. Not only are these low frequencies heard, but also they are felt.

Components of a Subwoofer Driver
The following diagram exhibits the cross-section of a subwoofer drive unit,

Subwoofers for home audio systems come in two basic types: powered (active) and unpowered (passive).

Active (Powered) Subwoofers
Powered subwoofers are by far the most popular, and generally the best option for most Home Theater applications. An active subwoofer is generally much more flexible. However, they do not necessarily sound the best. The simplest type of an active subwoofer would contain an amplifier, a crossover frequency control, a phase control, and some type of input connections. Since it contains an internal power amplifier, it will also need to be plugged into the wall power outlet.

The crossover frequency control allows you to set the range of frequencies that the subwoofer will reproduce and the frequencies it will filter out. The phase switch allows you to better integrate the subwoofer with the rest of the speakers in the system. It allows you to reverse the phase of the subwoofer’s audio signal. To set it properly, you’ll probably have to listen to it in both positions (0 and 180) to see which position creates the deepest bass in your home theater room. Finally, a powered subwoofer needs a line output from a receiver (or preamplifier).

Passive (Unpowered) Subwoofers
Passive subwoofers are designed to be powered by an external amplifier. The amplifier may be a dedicated amplifier (best option) or from the speaker terminals of the receiver. The important thing is that since a subwoofer needs more power to reproduce low frequency sound, the amplifier must be sufficiently powerful. In addition, if the subwoofer does not have a crossover frequency control, the signal to the subwoofer must be filtered by the receiver before it gets to the subwoofer. If the subwoofer has a a crossover network to filter out sound, it may also have output speaker terminals for the main or surround speakers.

For advantages and disadvantages bewteen active and passive subwoofers, check here.

Additional Subwoofer Characteristics
Auto on/off: This option allows the subwoofer to turn on when an input signal from the receiver or pre-amp is detected. The subwoofer will turn off in a few seconds or minutes (depending on the manufacturer) after there is no input.

Servo Feedback: Some advanced subwoofers may use a feedback signal from a device mounted on the speaker’s cone. The servo control unit compares the subwoofer’s output to the input signal and attempts to compensate for the driver’s output errors in order to smooth out distortion level. Unfortunately, this feedback circuit, no matter how quickly it works, cannot make the correction until after the error has occurred. As a result, the correction is always arriving at the subwoofer’s output with the next signal. Therefore, the correction is made on the wrong signal and it may compress the subwoofer’s transient response and remove its impact.

Down Firing: This type of subwoofer has the woofer installed in the bottom so that it fires toward the floor. Down-Firing subwoofers look like a piece of furniture (do not need a grill) and may be more efficient. It is important that these types of subwoofers are not placed in a corner too close to the walls as they may sound boomy.

Front Firing: This type of subwoofer has the woofer installed on the side and fires its output signal parallel to the floor. Front-Firing subwoofers need a grill to cover the woofer and look more like a speaker.

Sealed Enclosure: Originally this design was pioneered by companies like Acoustic Research. It consists of a driver mounted on one side of a sealed box. The air tight enclosure completely isolates the back wave of the driver from the front.

The sealed enclosure system is characterized by excellent transient response, excellent power handling at low frequencies, easy to design and build, smaller box size, and lower sensitivity to misaligned parameters when compared to port enclosure. However, sealed designs have a higher cutoff point and lower sensitivity than ported systems.

Because of excellent transient response (i.e., no boomy sound), when designed and build properly, some audiophiles prefer these type of subwoofers. There are others who completely dispute that sealed boxes have better transient response. They claim that the perception of transient is really a function of perceived sound quality and not the type of enclosure. According to these critics, what does improve transient response (or perceived quality) is usually more headroom, more drivers, larger boxes (depending on the driver), better efficiency, and very low distortion.

Ported Design: Some subwoofer enclosures may add an additional open port (sometimes called duct, vent, or tunnel) which allows the passage of air in and out of the box. At low frequencies, the port contributes up to +3dB to the output and makes the system more efficient and thus increases the bass response. A ported enclosure system consists of a driver mounted on one side of a box that has an open port (duct, vent, tunnel).

The ported subwoofers are characterized by lower distortion and higher power handling in the operating range, and lower cutoff frequency than a sealed enclosure system using the same driver. Distortion rapidly increases below the cutoff frequency however as the driver unloads and loses damping. Due to this, ported enclosures require a low frequency filter. The transient response of a ported enclosure is usually worse than a sealed enclosure system using the same driver. Ported enclosure systems are much more sensitive to misaligned parameters than sealed enclosure systems, which makes their construction more difficult.

Passive Radiator: Another type of subwoofer enclosure may add a passive radiator, instead of a port, to increase the efficiency of the sub. Passive radiators are sometimes drivers with the voice coil and magnet removed, or like a flat diaphragm. The radiator must usually be at least as large (or larger) than the driver in the enclosure.

Advantages of the passive radiator include the absence of port noise, and some audiophiles claim the radiator provides a better sounding bass than a ported enclosure. However, the cutoff (-3dB) frequency is slightly higher than ported design using the same driver.

Other Disadvantages include difficulty in tuning passive radiator designs as they have much higher mass than an equivalent port. In a port, the moving mass is comprised of the air in the port and the driver. The mass of the passive radiator, however, is high because it is the mass of the usually larger moving radiator plus the active driver. It is possible that when the powered subwoofer cone stops, the passive radiator's cone continues to move for a short period of time and may cause the powered driver cone itself to move after the signal has stopped. This so called "ping-pong" effect can cause distortion.

4th Order (Single Reflex) and 6th order (Dual Reflex) Bandpass Design: In a 4th Order Design the driver is completely buried in the enclosure and is mounted in a sealed chamber. It fires into a second ported chamber with the sound emanating from one or more ports.

These designs are very efficient within the operating bandwidth, with superior power handling, but generally can be very difficult to design and build. Companies like KEF helped pioneer this design.

The 6th Order Design was engineered and patented by Bose with their original AM-5 Acoustimass home speaker system.

This design is even more efficient than a single reflex bandpass, but with a compromise. Power handling within its frequency bandwidth is excellent, enabling these enclosures to play very loud. Transient response, however, is relatively poor, making them one of the most difficult enclosures to build and tune.

Size of the Driver: Subwoofers use the largest drivers (woofers) because they are responsible for moving the most air to create the lowest frequencies. Dropping down an octave in response requires four times as much output to maintain the same level. In general, the lower a speaker's resonance frequency, the lower the frequency reproducible by the speaker at a given level of distortion. Resonance frequency (denoted by Fs) is determined by a combination of the mass of the moving parts of the speaker (the cone, dust cap, voice coil, and former) and the compliance (i.e., flexibility) of the cone suspension (surround and spiders). Under normal conditions, we need a more powerful amplifier to drive a subwoofer. However, it is important to remember that you always want enough amplifier power to prevent clipping, but your main goal should be only to play the subwoofer at a level that blends with the rest of the system at any level.

The most popular sizes for subwoofers are 8", 10", 12", 15", or 18". Although an 18" subwoofer is capable of producing the lowest frequency bass at the highest volumes, a large driver is not necessarily the best option for optimum bass reproduction. Larger drivers are more difficult to control and tune. There are 10” subs on the market now that move as much air as some of the old 15” units. This is because the cone has a very large peak to peak excursion specification. It is important that the driver is designed correctly so that it stays in its linear range when moving this far. These subwoofers generally need a high power digital amplifier to make a long-throw woofer produce good performance in a small box.

Recommended Minimum Power for Subwoofers:
8" 50 watts
10" 75 watts
12" 100 watts
15" 150 watts
18" 250 watts

Room Size: For a smaller room, you can use subwoofers with smaller drivers. However, for bigger rooms, since there is more air volume for the subwoofer to pressurize, a 12" or 15" subwoofer is recommended. One of the biggest problems in home theater rooms is caused by standing waves. These are created when the wavelengths (or or wavelengths) of certain frequencies coincide with one or more room dimensions. Standing waves cause certain frequencies to be reinforced and cancelled at different locations throughout the room. The effect of standing waves is to have areas of the room where bass is very boomy and others where there is no bass at all. An equalizer will do nothing to fix these problems. These problem frequencies are known as room modes.

Ideally, two or more subwoofers may be a better option than a super large one, and since low-frequency sounds are non-directional, a subwoofer can be placed anywhere in the room. Please refer to the subwoofer placement section below.

Magnetic Shielding: Most modern speakers are magnetically shielded. Unshielded speakers can distort the picture and shift the colors. If an unshielded speaker must be placed close to a TV because of room-size or furniture considerations, it is recommended that you put two sheets of solid galvanized steel, cut to size, between the speaker and the TV to block the harmful electro-magnetic fields. Please note that magnetic fields are a major problem for CRT (tube) TVs. However, LCD and Plasma TVs are not affected by them.

Port Plugs: Port foam plugs allow customizing the subwoofer to suit your taste and room. They give the option that favor either maximum SPL output, or lower frequency bass extension depending on source material and preference. With port plug removed and the Port Mode switch set for "maximum output," output levels increase to room shaking levels. When the port plugs are installed, and the Port Mode switch is set to "maximum extension," the subwoofer is re-tuned for linear response to lower frequencies at a slightly reduced maximum output.

With home theater systems, ports are left open. This tuning provides an increase in bass output which is more ideal for movies where explosions and other action sounds need greater impact. For music applications, one or more of the ports is/are blocked. This tunes the subwoofer for a flatter response with extended low frequency response. It will produce lower frequencies and do so more accurately.

Speaker Impedance and Sensitivity
Read Impedance & Sensitivity of a Speaker.


Connecting the Subwoofer
If your receiver has a Subwoofer or LFE Output, run a standard RCA cable (RG6 cable is preferred) from the LFE Output on the receiver to the powered subwoofer’s LFE (Sub-In) input.

If your receiver does not have a Subwoofer or LFE Output, you can run two speaker wires from your receiver’s main speaker outputs to the subwoofer’s speaker inputs. Set the front speakers to Large and subwoofer to NO.

If your receiver does not have a Subwoofer or LFE output, but has an extra Pre-Outs, you can use a set of stereo RCA cables from the Pre-Outs of the receiver to the Left and Right Inputs on the powered subwoofer. In the receiver's menu, set the speakers corresponding to the Pre-Outs to Large.

Subwoofer Adjustment
Everyone wants the kind of bass they can feel, but what many people get is either weak or boomy bass. In some cases, they will have good bass at one seat location and little or no bass at other locations. Sometimes they try to crank up the subwoofer level to compensate for these deficiencies. Usually that makes the bass even more bloated and boomy. Sometimes it makes the amplifier clip or the subwoofer starts bottoming out.

A well-integrated subwoofer should not sound boomy. It should produce a deep and tight bass that blends with the main speakers. You should not be able to tell that the bass sound is coming from the subwoofer. This is particularly more important with music than movies.

Adjusting the Subwoofer Level
It is highly recommended that you use an SPL (Sound Pressure Level) meter to adjust the subwoofer level. If you do not have an SPL meter, use the built-in calibration program of your receiver or trust your ears. Read the Calibrating Your Audio with an SPL Meter thread.

Make sure you adjust the subwoofer channel level until you read the same number on the SPL meter (or hear the same level) as the main speakers. Because the low frequency sounds are very much room dependent, you should move around the listening area to get an average value.

For the most precise integration with your main speakers, go through test tones with an SPL meter. Setting the level using test tones by ear may result in misconfiguration. When you use the pink noise from your receiver to adjust the subwoofer level, you will not get accurate results. The main reason is that your calibration program or the SPL meter (if you are using one) measure the peak frequency generated by the sub at that listening position and all other frequencies are obscured. This is why it is best to use test tones and an SPL meter for a subwoofer calibration.

Using a test tone disc, adjust the volume so that the SPL meter reads 75dB with a 50Hz tone at the listening position. Do not play the test tones too loud as this may damage your speakers. Take measurements of four different tones 1/3 octave above 50Hz and four different tones 1/3 octave below the 50Hz. Average together each set of four measurements and adjust the subwoofer’s volume level to match the other speakers. Using the receiver's test tones is less accurate as they are not in one-third octave increments. Check the Using Test Tones thread.

Subwoofer Placement
Room acoustics and furniture have an enormous impact on the sound of speakers. A well placed subwoofer’s bass integrates with the sound of the main speakers and produces a natural reproduction of music. A few guidelines for subwoofer placement are listed below.

Corner placement: This is the advice that is given most often. Although corner placement will yield loud bass, it may make the music sound boomy. You should place your subwoofer in a corner only if it is not capable of producing deep bass. It is also important to note that corner placement will not always make the subwoofer boom. In most cases, it depends highly on the geometry of the room. According to some experts, you should always place a sub in a corner, and use equalization to deal with audible peaks of the subwoofer's frequency response at that position.

You should not sit against the wall: Your movies and music will sound heavy and tiring when you are sitting against a wall. If you must sit against the wall because of the room’s layout, turn down the volume of the subwoofer to compensate.

Do not place the subwoofer in a symmetrical position in the room: Avoid putting a subwoofer in a location that is the same distance from the walls. Subwoofers sound better if they are placed in a location where their distances to the front, side, and rear walls are different.

Put the subwoofer close to the main speakers: Even though bass sounds are not directional, you will get a better blending between the main speakers and the subwoofer if they are on the same side of the room.


Two or more subwoofers are better than one: According to some experts, the way multiple subwoofers interact with the room is the single biggest factor in being able to get great bass in every seat of your home theater. Depending on your budget, you should use either two or four subwoofers. There is not much benefit from using more than four. Multiple subwoofers can reinforce each other’s bass response and will yield a smoother and more dynamic sound. If using multiple subwoofers, you must use identical subwoofers. Different models, even from the same manufacturer, may cause uneven response.

When using two subs, they can be placed in the front corners of the room close to the main speakers or one can be placed in the front and the other one in the rear. For even better bass and smoother frequency response throughout your home theater, use four subwoofers. As a starting point, put the four subs at the midpoints of each wall.

It is possible that you may still get some peak points, but they can be taken care of with a good parametric (not graphic) equalizer. The problem with graphic equalizers is that most of them do not have better than 1/3 octave resolution. Although they can be quite expensive, parametric equalizers can give you much better results.

Setting the Low Pass Crossover Frequency
It is helpful to read A Guide to Crossover Networks for a better understanding of how crossover filters work.

You need to set the crossover on the receiver’s menu and not the subwoofer. If your main front speakers are full-size with good bass response, set the low pass filter to 80Hz. If your main speakers are small, bookshelf, satellite, or in-wall, set the low pass filter in the 100 to 120Hz range. According to the Recording Academy recommendations, selecting a frequency between 80 and 100Hz will produce the best results.

Run the test tone generator for intervals between 30Hz to 200HZ and listen to the output level or measure it with an SPL meter. If different output levels are heard or read by the SPL meter for different frequencies, it is quite normal as different frequencies interact differently with the room acoustics. Increase or decrease the low-pass frequency to achieve the smoothest response. Decrease the crossover frequency if there is too much output around the crossover point, increase it if you notice a drop in the response.

In general, if the main speakers are large and capable of creating low frequency sound, it will be easier to match them with a subwoofer as opposed to the smaller speakers that most people own. However, it is not always best to set the crossover frequency at the lowest possible frequency to avoid the sonic signature of the subwoofer. If the crossover is set as low as possible, the subwoofer cannot stimulate the main speaker drivers near their resonance frequency, leaving only the main speakers as the dominant resonance contributor. If the crossover is set slightly higher, the bass from the subwoofer and the main speakers can reinforce one another resulting in a smoother and more pleasing bass.

Setting the Subwoofer’s Phase (Polarity)
When the woofer on the sub and the woofers of the front speakers move in and out in sync with each other, the system is said to be in phase. When the speakers and the subwoofer are moving out of sync with each other, the subwoofer and the front speakers’ bass overlap and cancel each other. In this case the system is said to be out of phase, resulting in less bass.

Unfortunately, there may be another problem between the main speakers and the subwoofer. If the main speakers are producing bass at the same time as the subwoofer, at some points the bass will reinforce each other. At other points the bass will cancel each other. The solution is to allow only the subwoofer to reproduce bass by setting the front speakers to small in the receiver’s setup menu. This can yield a smoother bass response throughout the entire room.

To get the best bass response, you should set the phase (polarity) of the subwoofer(s) to deliver the highest output at the listening position. This can be achieved with the help of a test signal at the crossover frequency and an SPL meter. You should run this test several times by changing the polarity of the subwoofer and measuring the bass response on the SPL meter. Select the phase option that results in the highest bass response. If you don’t have an SPL meter, you will have to trust your ears.

Fortunately, most subwoofers have a switch to change their polarity. If the subwoofer does not have a phase switch, you can change the polarity of the main speakers by switching the positive with the negative speaker wires (the black wire goes to the red terminal and the red wire to the black terminal). Some subwoofers have a “variable phase control”. This control can be set continuously between 0 and 180 and allows for a more precise phase control of the subwoofer.

When setting the subwoofer phase by ear, play some music (not a movie) that has a repetitive bass line. Switch the polarity several times and choose whichever setting sounds “faster” or “fuller”. If you do not hear any difference, leave the phase switch at “0" or “normal”.

If you are using two subwoofers, you have to position them properly and run the experiment mentioned above by adjusting the phase of one subwoofer and observing the result on the SPL meter. You may possibly have to set the phase of one of the subwoofers to 180.

Importance of Polarity (Phase)
The animation in the following shows two waves traveling in the same direction. The phase difference between the two waves varies with time so that we see constructive interference when maximum points are aligned (peak) and destructive interference when minimum and maximum points are aligned (null). This illustrates why it is important to adjust the subwoofer’s phase control with respect to the main speakers so that we obtain maximum output.

How to Check the Polarity of Speakers: Take the grill off a speaker. Apply a wire from the (-) terminal on the speaker to the (-) end of a AA battery. Then touch a wire from the (+) terminal on the speaker to the (+) end of the battery and look at the cone of the speaker. The speaker cone should move forward. If the cone moves backward, the speaker is wired out of phase. In some instances midrange drivers are intentionally wired out of phase with their woofers.

Repeat Everything Several Times
All the speaker adjustments are inter-dependent. Once you change something like polarity, you may have to go back and check the low-pass crossover frequency. Getting the optimum bass response from a subwoofer is a tricky business. There are no easy solutions. Don’t be afraid to experiment. If you hear differently than the SPL meter, trust your ears and not the meter. After all, we are not bats or dogs! To read about the hearing range differences between animals and humans, read the following: .



Check the following sites for a list of movies with good bass sound to test your subwoofer.



Last edited by Big Daddy; 02-06-2013 at 06:24 AM.
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